For example, an electric vehicle using an electric energy as a power source is provided with a battery pack consisting of plural batteries so that it is supplied with necessary power. For the plural batteries contained in the battery pack, it is required to uniform a voltage of each battery so as to improve stability and span thereof and to obtain a high power. The electric vehicle is provided with a plurality of battery packs depending on vehicle structures, each of packs consisting of 10 batteries. A battery management system (BMS) charges or discharges the batteries of the battery pack to enable each of batteries to have an appropriate voltage. However, since it is difficult for the plural batteries to stably maintain an equilibrium state due to various causes such as change of an internal impedance, etc., a balancing function for equilibrating charged states of the plural batteries is provided to the battery management system.
For doing so, according to the prior art, in order to uniform voltages of the batteries in a high voltage battery pack using the plural batteries, a discharge resistance (buck resistance) is connected to a battery exhibiting a higher voltage than a mean voltage so that energy of the battery is consumed through the resistance, thereby reducing a difference of voltages in the whole battery pack. This method performs the discharge in an on/off manner by a control circuit. However, according to this method, as the discharge is carried out, the battery voltage is dropped. If a voltage is dropped and a resistance is fixed, a current capacity is also dropped due to Ohm's law (V=IR). Accordingly, as the discharge proceeds, an effect of the discharge is also decreased. In addition, in case of using a solid state relay (SSR), since there are limitations in the maximum current capacity allowable in the SSR, an initial discharge is carries out with a low current, so that it takes much time to perform the discharge. In addition, since the time for performing the discharge is extended, it is not possible to perfectly complete the discharge operation within the limited time.
FIG. 1 shows a battery voltage, a battery current and a discharge energy when a battery is conventionally discharged. In the conventional discharge method, a discharge target battery and a discharge resistance are connected to drop the voltage of the discharge target battery. At this time, as shown in FIG. 1, it can be seen that the discharge effect is reduced as time goes by, since the discharge resistance is fixed.
For example, provided that the highest voltage of the battery is 4.5V and a consumption power of the discharge resistance is 1 W, the current (I) is about 220 mA (P=VI). Accordingly, the resistance (R) is determined to be 22.5 (V=IR). In this case, in the method of using the fixed resistance as the prior art, if the determined resistance (R=22.5) is used when the voltage of the battery becomes 2.5V, the flowing current is about 110 mA (V=IR).
At this time, the discharge power P consumed by the resistance is about 0.275 W (P=VI).
In other words, since the power is consumed in the discharge resistance, the voltage and the current are decreased as time goes by, so that the energy consumed by the discharge resistance is also reduced.
As described above, according to the prior art, the discharge effect is decreased as the discharge performing time lapses, so that it is delayed the reduction in the battery voltage and thus the discharge cannot be perfectly carried out within the limited time.